Protection circuit that can be associated with a filter

ABSTRACT

A structure for protection against electrostatic surges having two input terminals and two output terminals. The output terminals of the structure are connected to the inputs of a circuit to be protected. A first input terminal is connected to a first output terminal via an impedance. The second input terminal is connected to the second output terminal. The input terminals are interconnected by a first avalanche diode. The output terminals are interconnected by a second avalanche diode having the same biasing as the first avalanche diode.

This application is a Continuation-in-Part of application Ser. No.09/162,399, filed Sep. 28,1998 U.S. Pat. No. 6,147,853, entitledPROTECTION CIRCUIT THAT CAN BE ASSOCIATED WITH A FILTER and now pending,which application claims the benefit of priority under 35 USC §119(a) toFrench Application No. 97 12297, filed Sep. 29, 1997.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of protection circuits thatprotect against electric surges that may affect components and/orelectronic devices such as, for example, mobile phones, computers,printers. It is particularly desired to protect such devices againstelectrostatic surges which can, for example, correspond to currentpeaks, that have short durations but very high intensities, for example,several tens of amperes.

2. Discussion of the Related Art

Conventionally, as is shown in FIG. 1A, to protect inputs 1 and 2 of adevice 3 connected to external components, an avalanche diode 5 or anequivalent device is disposed between these inputs. Resistors 7 and 8correspond to connection resistances or to resistors voluntarilyinserted in the circuit.

Theoretically, as is shown in FIG. 1B, an avalanche diode requires avoltage V_(Z) between its terminals to be conductive. However, when thecurrent through this diode significantly increases, as is shown in FIG.1B, voltage V across the avalanche diode increases to reach a valueV_(S) equal to V_(Z)=R_(Z)I, where I is the current associated with thesurge. To take a practical example, considering an avalanche diode of asurface area of approximately 0.4 mm², in conventional technologies, theequivalent dynamic resistance R_(Z) will be on the order of 0.4 ohm. Ifthe current peak is on the order of 30 amperes, the effective voltageacross an avalanche diode having a breakdown voltage of 6 volts will be6+0.4×30=18 volts. The effective value is triple the nominal voltage anda destruction of the components of the circuit to be protected canresult therefrom.

It is known that to reduce the dynamic resistance of an avalanche diode,its surface should be increased. If the surface is increased by a factor4, to reach for example a surface of 1.6 mm², the equivalent resistancewill be of 0.1 ohms only. The overvoltage linked to the current flowwill be of 3 volts only, that is, the voltage across the avalanche diodewill reach, for 30 amperes, a value on the order of 9 volts, whichremains acceptable.

However, this has the disadvantage that the cost of a diode increaseswith its surface and that a significant surface has to be provided sothat the increase of the voltage with the current becomes negligible.

Further, an avalanche diode inevitably exhibits in the blocked(non-conducting) state a certain stray capacitance. In a conventionalcase, for a diode of 0.4 mm², there will be a stray capacitance on theorder of 250 pF. If the diode surface is multiplied by 4, the strayecapacitance will be multiplied by four to reach a value on the order ofone nanofarad. As a result, the protection system exhibits a highcapacitance which cannot be chosen independently from the choice of theresistance. This can be a disadvantage in some applications.

SUMMARY OF THE INVENTION

Thus, an object of the present invention is to overcome thedisadvantages of prior art systems such as described hereabove.

Another object of the present invention is to provide a protectionstructure such that the voltage thereacross remains close to theexpected nominal value.

Another object of the present invention is to provide such a structurewhich occupies a reduced silicon surface.

Another object of the present invention is to provide such a structurewhich exhibits a selected filtering characteristic.

To achieve these and other objects, the present invention provides astructure for protection against electric surges, connected between twoinput terminals and two output terminals, the output terminals beingconnected to the inputs of a circuit to be protected, a first inputterminal being connected to a first output terminal via an impedance,the second input terminal being connected to the second output terminal,the input terminals being interconnected by a first avalanche diode, theoutput terminals being interconnected by a second avalanche diode ofsame polarity as the first avalanche diode.

According to an embodiment of the present invention, the two avalanchediodes are identical or at least substantially identical.

According to an embodiment of the present invention, the two avalanchediodes are bidirectional.

According to an embodiment of the present invention, the impedance is aresistor connected between the first input and output terminals.

According to an embodiment of the present invention, the impedance is atripole impedance, a third terminal of which is connected to the secondinput and output terminals, the elements of the tripole forming, withthe avalanche diodes, a filtering structure.

According to an embodiment of the present invention, the tripoleincludes two series resistors and a parallel capacitor.

The present invention also provides monolithic implementations of theabove structure. In one embodiment a monolithic implementation of theimplementation of the protection structure includes, in a substrate of afirst type of conductivity, two areas of a second type of conductivityforming Zener junctions with the substrate, and one metallization formedon an upper surface of the substrate with interposition of an insulatinglayer.

In another embodiment, a monolithic implementation of the protectionstructure is formed from a substrate of a first conductivity type, thesubstrate having a front surface and a rear surface. The structureincludes a region of a second and opposite conductivity type formed onthe front surface of the substrate. The region of the second andopposite conductivity type has three main areas, each of the three mainareas being covered with a respective first metallization. Two of thefirst metallizations are respectively connected to one of the firstinput terminal and the first output terminal, and the structure furtherincludes a second metallization covering the rear surface of thesubstrate.

According to an advantage of the present invention which will bediscussed in further detail hereafter, the structure of the presentinvention is particularly well adapted to being combined with structuresensuring determined filtering effects.

The foregoing objects, features and advantages of the present invention,will be discussed in detail in the following non-limiting description ofspecific embodiments in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a protection structure according to prior art;

FIG. 1B shows the electric current-voltage characteristic of theprotection structure of FIG. 1A;

FIG. 2 shows an example of embodiment of a protection structureaccording to the present invention;

FIG. 3 shows a general shape of the protection structure according tothe present invention;

FIGS. 4A, 4B, 4C show various alternatives of the protection structureaccording to the present invention; and

FIG. 5 shows a first implementation in the form of a monolithicstructure of the protection circuit of FIG. 4C; and

FIGS. 6A and 6B show a second implementation in the form of a monolithicstructure of the protection circuit of FIG. 4C.

DETAILED DESCRIPTION

As is shown in FIG. 2, a protection structure 10 according to thepresent invention includes input terminals 11 and 12 and outputterminals 13 and 14 connected to the circuit 3 to be protected.Terminals 12 and 14 are interconnected. Terminals 11 and 13 areconnected via a resistor R. Terminals 11 and 12 are connected via anavalanche diode Z1. Terminals 13 and 14 are connected via an avalanchediode Z2.

Assuming that avalanche diodes Z1 and Z2 are identical and have the sameavalanche voltage V_(Z) and the same dynamic resistance R_(Z), the inputcurrent I_(s) resulting from a surge is:

I _(S) =I ₁ +I ₂  (1)

where I₁ designates the current in avalanche diode Z1 and I₂ designatesthe current in avalanche diode Z2.

Neglecting the current coming out of terminal 13, the following relationcan be written:

I ₁ R _(Z) =I _(Z)(R+R _(Z)).  (2)

This leads to: $\begin{matrix}{I_{S} = {{I_{1}( {1 + \frac{R_{Z}}{R + R_{Z}}} )} = {I_{1}( \frac{R + {2R_{Z}}}{R + R_{Z}} )}}} & (3) \\{I_{S} = {{I_{2}( {1 + \frac{R + R_{Z}}{R_{Z}}} )} = {I_{2}( \frac{R + {2R_{Z}}}{R_{Z}} )}}} & (4)\end{matrix}$

Thus, the input voltage V_(e) resulting from an overload can be writtenas: $\begin{matrix}{V_{e} = {V_{Z} + {R_{Z}I_{S}\quad \frac{R + R_{Z}}{R + {2R_{Z}}}}}} & (5)\end{matrix}$

and the output voltage then is: $\begin{matrix}{V_{S} = {V_{Z} + {R_{Z}I_{S}\quad \frac{R_{Z}}{R + {2R_{Z}}}}}} & (6)\end{matrix}$

Equation (6) shows that if value R is much higher than value R_(Z),output voltage V_(S) is substantially equal to V_(Z).

In a specific example where resistance R_(Z) is, as previously, equal to0.4 ohm and where R=50 ohms, factor R_(Z)/(R+2R_(Z)) is equal to0.4/50.8, that is, approximately 1/100.

Thus, by using for diodes Z1 and Z2 the same diode as diode 5 of FIG.1A, an overvoltage of 0.1 volt only results from the current flow,instead of a 12-volt overvoltage. This shows that, while keeping a veryreasonable overvoltage, the surface of the avalanche diodes can befurther decreased to further reduce the stray capacitance of theparallel diodes. Diodes four times as small, each having a straycapacitance on the order of 70 pF can, for example, be used.

Those skilled in the art should note that the structure of FIG. 2 canhave various alternatives, the two diodes Z1 and Z2 not beingnecessarily identical, but being adaptable to specific cases.

More generally, protection circuit 10 according to the present inventioncan be such as shown in FIG. 3, resistor R being replaced with a tripoleimpedance Z. The previously described calculations apply, assuming thatvalue R is the series resistance of tripole Z.

Different specific examples of the general diagram of FIG. 3 are shownas an example in FIGS. 4A to 4C.

In FIG. 4A, an assembly of a resistor R2 and of an avalanche diode Z3has been used in addition to avalanche diodes Z1 and Z2 and to resistorR (designated as R1 in FIG. 4A). Thus, if the first assembly provides animprovement factor 1 of 100 over the overvoltage introduced by thecurrent overload, complementary assembly R2-Z3 will provide anadditional improvement factor of 100, that is, a voltage variation whichis on the order of {fraction (1/10,000)} only. This assembly can be usedin cases where the series disposition of two resistors R1 and R2 bearsno prejudice to the circuit operation and ensures, especially by thechoice of the capacitance of diode Z2, a low-pass filter function.

FIG. 4B shows a structure in which tripole Z includes two resistors R1and R2 in series between terminals 11 and 13 and one resistor R3 betweenthe connection node of resistors R1 and R2 and terminals 12, 14. Anadvantage of this structure is to insert an additional bridge fordividing the overvoltage.

In the case of FIG. 4C, impedance Z is a tripole including two resistorsR1 and R2 in series between terminals 11 and 13 and a capacitor C1between the connection node of resistors R1 and R2 and terminals 12, 14.

As will be noted by those skilled in the art, the assembly forms alow-pass filter and for example eliminates the carrier wave in a mobilephone. This filter has cut-off frequencies f₁=1/RC₁ω andf_(Z)=1/RC_(Z)ω, where C_(Z) designates the capacitance of an avalanchediode, a first attenuation appearing for the first cut-off frequency anda stronger attenuation appearing for the second cut-off frequency.

For R1=R2=R=250Ω, C₁=25 pF, and C_(Z)=70 pF, one obtains f_(Z)=45 MHZand f₁=64 MHZ.

For R1=R2=R=50Ω, C₁=50 pF, and C_(Z)=70 pF, one obtains f=9 MHZ andf₁=25 MHZ.

Thus, the present invention combines, in a simple manner, a protectionstructure and a filtering structure.

FIG. 5 shows a monolithic implementation of the structure of FIG. 4Caccording to one embodiment of the present invention. In the exampleshown, the structure is formed in a P-type substrate 20 in which areformed two N-type regions 21 and 22 forming Zener junctions with thesubstrate. The lower surface of the component is coated with ametallization M1 corresponding to terminals 12 and 14 of FIG. 4C.Metallizations M2 and M3 formed on regions 21 and 22 correspond toterminals 11 and 13. A capacitor can be implemented between ametallization M4 and substrate 1 and corresponds to capacitor C1 of FIG.4C. Resistors R1 and R2 can be formed in any known manner, for example,by providing thin conductors forming a portion of the metallization ofwhich are formed contacts M2, M3, and M4. Resistors may also be providedin a polysilicon layer, or diffused resistors formed in wells themselvesformed in substrate 20 may further be provided.

As previously indicated, the total surface of junctions 21 and 22 can bevery small.

Of course, the structure of FIG. 5 and the circuits illustrated as anexample in FIGS. 4A to 4C are likely to have various alterations,modifications, and improvements which will readily occur to thoseskilled in the art. Further, although the present invention has beendescribed with a one-way (unidirectional) protection component, it ofcourse also applies to bidirectional protection components.

FIGS. 6A and 6B illustrate a second exemplary implementation of of thecircuit of FIG. 4C with FIG. 6B being a sectional view of the circuit ofFIG. 6A taken along line B—B. The component is made of a P-typesubstrate 30 that includes on its front or upper surface a N-type region31. On the rear or lower surface is formed a metallization M11,preferentially with interposition of a more heavily doped P-type region32. On the front surface are formed respective metallizations M12, M13,M14 in contact with areas of the region 31. Specifically, metallizationM12 is formed over an area substantially corresponding to the Zenerdiode Z1, and metallization M13 is formed over an area substantiallycorresponding to the capacitor C1.

As better shown in FIG. 6A, the diffused region 31 preferentiallyincludes portions 31-1 and 31-2 that are not covered with ametallization and that extend between the area covered by metallizationM14 and, respectively, the area covered by metallization M12 and thearea covered by metallization M13. The portions 31-1 and 31-2respectively correspond to resistors R1 and R2.

Although it has been indicated above that each component is formed undereach metallization, it will be clear for those skilled in the art that,in fact, capacitor C1 is distributed and extends further than the areaimmediately under the metallization M14, under the extensionscorresponding to the portions 31-1 and 31-2. It will also be noted thatone may consider the area disposed under the metallization M14 as aZener diode or as a capacitor but that, in the desired operation, thisarea behaves like a capacitor that has the property of being“self-protected” against electrostatic discharges.

In one exemplary implementation, the P-type substrate 30 has a dopinglevel of about 10¹⁸ atoms/cm³ (resistivity of about 10 mΩ/cm). In thisexemplary implementation, N-type region 31 has a doping level of about10²⁰ atoms/cm³, which corresponds to a resistance of about 10 Ω/square.

An advantage of the structure of FIGS. 6A-6B is that it is simpler tomanufacture than the structure of FIG. 5. Indeed, it is not necessary toprovide an oxide layer having a well determined thickness to form thedielectric of a capacitor having a desired capacitance. Also no specialsteps have to be provided for implementing the resistors R1 and R2.

Another advantage of the structure of FIGS. 6A-6B is that the value ofthe capacitor is simply adjusted by the mask delimiting the surface areaof region 31 under metallization M14.

Another advantage of the structure of FIGS. 6A-6B is that its surfacearea can be smaller for a desired capacitance value than the structureof FIG. 5. This is due to the fact that a junction capacitance has avalue of about 1500 pF/mm² while a dielectric capacitance, for an oxidethickness of 100 nm, will have a capacitance of only about 500 pF/mm².In the current power technologies, it is difficult to go under 100 nmfor the dielectric thickness.

Of course, this second embodiment is also capable of variousmodifications that will appear to those skilled in the art. For example,one could provide other implementations of the resistors than thatdescribed with respect to FIGS. 6A and 6B. For example, the resistorsmay be formed of thin metallizations or polysilicon on an insulatingmaterial. In this case, the areas of region 31 formed under eachmetallization would be separated. Additionally, it will be noted that ispossible with a single diffused structure to obtain resistances ofadjustable value by covering with a metallization at least a part of theportions 31-1 and 31-2.

Such alterations, modifications, and improvements are intended to bewithin the spirit and the scope of the present invention. Accordingly,the foregoing description is by way of example only and is not intendedto be limiting. The present invention is limited only as defined in thefollowing claims and the equivalents thereto.

What is claimed is:
 1. A protection circuit for protection againstelectric surges, connected between two input terminals and two outputterminals, the two output terminals being adapted for connection toinputs of a circuit to be protected, wherein: a first input terminal ofthe two input terminals is connected to a first output terminal of thetwo output terminals via a tripole impedance; a second input terminal ofthe two input terminals is connected to a second output terminal of thetwo output terminals; the first input terminal is interconnected to thesecond input terminal across a first avalanche diode; the first outputterminal is interconnected to the second output terminal across a secondavalanche diode of a same polarity as the first avalanche diode; thetripole impedance includes two series connected resistors and acapacitor, a first terminal of the capacitor being connected to the twoseries connected resistors and a second terminal of the capacitor beingconnected to the second input terminal and the second output terminal,elements of the tripole impedance forming, with the first and secondavalanche diodes, a filtering structure; and wherein the protectioncircuit is implemented monolithically in a substrate of a firstconductivity type, the substrate having a front surface and a rearsurface, the structure including a region of a second and oppositeconductivity type formed on the front surface of the substrate, saidregion having three main areas, each of the three main areas beingcovered with a respective first metallization, two of the firstmetallizations being respectively connected to one of the first inputterminal and the first output terminal, the structure further includinga second metallization covering the rear surface of the substrate. 2.The protection circuit of claim 1, wherein one of the three main areasis connected to each of the other two main areas by a portion of saidregion of the second conductivity type having a smaller surface areathan portions of said region of the second conductivity type that arebelow the first metallizations covering the other two main areas.
 3. Theprotection circuit of claim 1, wherein the two main areas arranged underthe two first metallizations have a same surface area.
 4. The protectioncircuit of claim 1, wherein the substrate of the first conductivity typeis a P-type substrate having a doping level of approximately 10¹⁸atoms/cm³ and said region of the second and opposite conductivity typeis a N-type region having a doping level of about 10²⁰ atoms/cm³.